In a manganese battery, an alkaline battery, a nickel-hydride battery, a lithium battery, a lithium ion secondary battery, and the like, a particulate material is used as an active material for storing electricity, and a binder is necessary to bind such particles. A general binder is a polymer organic compound with extremely low conductivity. Therefore, a conduction auxiliary agent such as acetylene black, graphite particles, or carbon fibers is mixed into the material so as to increase the conductivity (see Patent Document 1).
Specifically, active material particles, the conduction auxiliary agent, and the binder are mixed, and the mixture is applied onto a current collector, molded, and then dried to be used as an electrode such as a positive electrode or a negative electrode. A similar procedure is applied to other electric appliances including a particulate material, without limitation to a battery.
In the case where the conductivity of active material particles themselves is low, it is necessary to add a larger amount of conduction auxiliary agent or to form a conductive film using carbon or the like over the surfaces of the active material particles (to carbon coat). Further, in the case where the ion conductivity of the active material particles is low in a power storage device utilizing ion conductivity (e.g., lithium ion secondary battery), it is necessary to use active material particles with a small particle size.
For example, lithium cobaltate has been used as a positive-electrode active material in a lithium ion secondary battery. Lithium cobaltate is preferably used as a positive-electrode active material in a lithium ion secondary battery because of its relatively high conductivity and ion conductivity. However, cobalt which is a material has modest deposits and is produced in limited regions, and thus has a problem in terms of price and stable supply.
In contrast, iron is inexpensive due to its abundant production, and Non-Patent Document 1 discloses that lithium iron phosphate which is obtained by using iron can serve as a positive electrode material of a lithium ion secondary battery. Lithium iron phosphate, however, has lower lithium ion conductivity and electric conductivity than lithium cobaltate, and thus needs to be carbon coated and be microparticles with an average particle size of 150 nm or less, preferably 20 nm to 100 nm. Note that the particle size is the size of a primary particle.
However, since such microparticles are likely to aggregate, it is difficult to mix lithium iron phosphate particles and a conduction auxiliary agent uniformly. To prevent the particles from aggregating, the proportion of the conduction auxiliary agent needs to be increased, but the increase makes it difficult to maintain the form of an electrode and the proportion of a binder also needs to be increased, resulting in a reduction in storage capacity.
In the case where graphite particles are used as the conduction auxiliary agent, natural graphite is generally used by reason of cost. However, in that case, iron, lead, copper, or the like contained in the graphite particles as an impurity reacts with an active material or a current collector, so that the potential and the capacity of the battery are decreased.
Acetylene black contains fewer impurities and has a better developed chain structure than graphite particles and therefore has excellent electrolyte retention characteristics, thereby improving the use efficiency of an active material. However, since a particle of acetylene black is a microparticle with a diameter of about 10 nm, current is conducted from the lithium iron phosphate particles by hopping between individual acetylene black particles or acetylene black particle groups.
That is, every time the hopping occurs, the resistance is increased and the discharging voltage is decreased when the power storage device releases electricity, i.e., a voltage drop occurs. The above problem is also caused in the case where graphite particles are used. FIG. 2A illustrates a schematic cross-sectional view of an electrode including acetylene black as a conduction auxiliary agent.
As described above, microparticles of active material particles are likely to aggregate and unlikely to be mixed with a binder or acetylene black uniformly (or to be dispersed in a binder uniformly). Therefore, a portion where active material particles are concentrated (portion where the active material particles aggregate) and a portion where active material particles are thinly distributed are generated, resulting in a reduction in the proportion of the active material in the electrode. Further, the portion where the active material particles are concentrated includes a portion where acetylene black or the like does not exist, so that the conductivity in that portion is low and an active material that cannot contribute to capacity is generated.
FIG. 2B shows a SEM image of a positive electrode of a conventional ion secondary battery. In a general conventional electrode, the proportion a material other than the active material has been 15% or higher. To increase the capacity of a battery, it is necessary to reduce the weight or volume of the material other than the active material. It is also necessary to take measures to prevent the material other than the active material (especially a binder) from swelling because the swelling might cause deformation or breakdown of the electrode.